1. Field of the Invention
The present invention relates to an optical recording medium having a plurality of information recording planes for reproducing information by laser beam having a wavelength of 400 nmxc2x15 nm.
2. Description of the Related Art
An optical recording medium having a plurality of information recording planes is realized today. Such recording medium, for example, a DVD (digital versatile disk) has two or four information recording planes, and the information of two information recording planes can be read without inverting the disk in the former type. In the latter type, by inverting the disk, the information of two information recording planes can be read out from both sides of the disk.
Incidentally, Japanese Laid-open Patent No. 11-283278 discloses an optical recording medium having three information recording planes capable of reading from one side.
From the viewpoint of enhancing the recording density in the optical recording medium, and enhancing the convenience of use, it is effective to increase the number of information recording planes that can be read from one side.
It is hence an object of the invention to present an optical recording medium having four or more information recording planes capable of reading from one side.
The optical recording medium of the invention is an optical recording medium (100) comprising at least four information recording planes and (first to fourth) reflective layers (3, 5, 7, 9) formed on first to fourth information recording planes (3a, 5a, 7a, 9a) respectively, in which the first reflective layer (3) has a layer mainly composed of titanium oxide, the second reflective layer (5) has a layer mainly composed of titanium oxide, the third reflective layer (7) has a layer made of an alloy mainly composed of silver or a layer laminating the silver alloy layer with the titanium oxide layer, and the fourth reflective layer (9) has a layer made of an alloy mainly composed of aluminum.
According to the invention, since the first reflective layer (3) has a layer mainly composed of titanium oxide, the second reflective layer (5) has a layer mainly composed of titanium oxide, the third reflective layer (7) has a layer made of an alloy mainly composed of silver or a layer laminating the silver alloy layer with the titanium oxide layer, and the fourth reflective layer (9) has a layer made of an alloy mainly composed of aluminum, variations of reflected light quantity from each reflective layer can be suppressed. The information on the first to fourth information recording planes (3a, 5a, 7a, 9a) can be read from one side of the optical recording medium by using laser beam having a wavelength of 400xc2x15 nm. The invention also comprises other reflective layer than the first to fourth reflective layers (3, 5, 7, 9), that is, it is applicable also to an optical recording medium having five or more reflective layers. The invention is further applicable to a play-only type optical recording medium. One or arbitrary plural layers of the first to fourth reflective layers (3, 5, 7, 9) may be formed in a laminated structure. The first to fourth information recording planes (3a, 5a, 7a, 9a) are formed on each plane of the first to fourth transfer layers (2, 4, 6, 8) touching with the first to fourth reflective layers (3, 5, 7, 9), respectively. The transfer layers may be formed as cured article of liquid or semisolid plate of photosetting resin such as polyacrylate resin, polymethacrylate resin and the like. As a quality of material of the transfer layers, thermosetting resin such as polycarbonate resin and the like may be used, as well. The information recorded in the optical recording medium can be supported in undulations, that is, phase pits of the first to fourth information recording planes (3a, 5a, 7a, 9a).
The titanium oxide in the first and second reflective layers (3, 5) is preferably composed of oxygen/titanium at a ratio of 1.8 or more to 2.2 or less. In this case, since an optical characteristic close to that of titanium dioxide (TiO2) can be obtained, the reflectivity and transmissivity of the first and second reflective layers (3, 5) can be controlled adequately in particular.
The layer mainly composed of titanium oxide in the first and second reflective layers (3, 5) is preferably either amorphous or fine crystal composition. In this case, the first and second reflective layers (3, 5) are homogeneous optically. The amorphous or fine crystal composition is same as amorphous composition optically.
The thickness of the first and second reflective layers (3, 5) is preferably 80 nm or less. The lower limit of the thickness may not necessarily be limited as long as the layers can be formed uniformly. Practically about 10 nm or so may be a lower limit. In this case, the reflectivity and transmissivity of the first and second reflective layers (3, 5) can be controlled adequately in particular.
The alloy mainly composed of silver in the third reflective layer (7) preferably contains either palladium or ruthenium, and either copper or titanium, and the ratio of silver in the composition is 94 percent by weight or more. In this case, the reflectivity and transmissivity of the third reflective layer (7) can be controlled adequately in particular.
The thickness of the third reflective layer (7) is preferably 20 nm or less. The lower limit of the thickness may not necessarily be limited as long as the layers can be formed uniformly as mentioned above. In this case, the reflectivity and transmissivity of the third reflective layer (7) can be controlled adequately in particular.
The third reflective layer (7) is preferably formed by laminating the layer of alloy mainly composed of silver, and a layer mainly composed of titanium oxide. In this case, since the absorption rate of the third reflective layer (7) can be decreased, the reflectivity and transmissivity can be controlled adequately in particular.
The titanium oxide in the third reflective layer (7) is preferably composed of oxygen/titanium at a ratio of 1.8 or more to 2.2 or less. In this case, the reflectivity and transmissivity of the third reflective layer (7) can be controlled adequately in particular.
The layer mainly composed of titanium oxide in the third reflective layer (7) is preferably either amorphous or fine crystal composition. In this case, the third reflective layer (7) is homogeneous optically. The amorphous or fine crystal composition is same as amorphous composition optically.
The thickness of the layer mainly composed of titanium oxide in the third reflective layer (7) is 80 nm or less. The lower limit of the thickness may not necessarily be limited as long as the layers can be formed uniformly as mentioned above. In this case, the reflectivity and transmissivity of the third reflective layer (7) can be controlled adequately in particular.
The alloy mainly composed of aluminum in the fourth reflective layer (9) preferably contains at least one of titanium, chromium, zinc, manganese, and copper, and the ratio of aluminum in the composition is 90 percent by weight or more. In this case, the reflectivity of the fourth reflective layer (9) has a larger value.
The thickness of the layer of alloy mainly composed of aluminum in the fourth reflective layer (9) is 20 nm or more. In this case, the reflectivity of the fourth reflective layer (9) has a larger value.
The ratio of maximum and minimum is preferably 2 times or less, among the reflected light quantity from the first reflective layer (3), the reflected light quantity from the second reflective layer (5) passing through first reflective layer (3), the reflected light quantity from the third reflective layer (7) passing through the first reflective layer (3) and second reflective layer (5), and the reflected light quantity from the fourth reflective layer (9) passing through the first reflective layer (3), second reflective layer (5), and third reflective layer (7).
In this case, since the information recorded corresponding to each reflective layer can be read accurately, a practical optical recording medium having four layers of information recording planes can be obtained. When applied in an optical recording medium having other reflective layer than the first to fourth reflective layers (3, 5, 7, 9), that is, having five or more reflective layers, the ratio of maximum and minimum of reflected light quantities from five reflective layers is set at 2 times or less.
The optical recording medium of the invention is also an optical recording medium (100) comprising first to fourth reflective layers (3, 5, 7, 9) formed on first to fourth information recording planes (3a, 5a, 7a, 9a) respectively, in which the ratio of maximum and minimum is preferably 2 times or less, among the reflected light quantity from the first reflective layer (3), the reflected light quantity from the second reflective layer (5) passing through first reflective layer (3), the reflected light quantity from the third reflective layer (7) passing through the first reflective layer (3) and second reflective layer (5), and the reflected light quantity from the fourth reflective layer (9) passing through the first reflective layer (3), second reflective layer (5), and third reflective layer (7).
According to the invention, since variations are suppressed so that the ratio of maximum and minimum of reflected light quantities from each reflective layer may be 2 times or less, the information recorded corresponding to each reflective layer can be read accurately. Therefore, a practical optical recording medium having four layers of information recording planes can be obtained. As far as the reflectivity and transmissivity of each reflective layer can be adequately controlled, the material is not particularly limited. The information on the first to fourth information recording planes (3a, 5a, 7a, 9a) can be read from one side of the optical recording medium by using laser beam. The invention also comprises other reflective layer than the first to fourth reflective layers (3, 5, 7, 9), that is, it is applicable also to an optical recording medium having five or more reflective layers. In this case, the ratio of maximum and minimum of reflected light quantities from five reflective layers is set at 2 times or less. The invention is further applicable to a play-only type optical recording medium. The first to fourth information recording planes (3a, 5a, 7a, 9a) may be formed as cured matter of liquid photosetting resin or cured matter of semisolid flat photosetting resin. The information recorded in the optical recording medium can be supported in undulations, that is, phase pits of the first to fourth information recording planes (3a, 5a, 7a, 9a).
For the ease of understanding of the invention, reference numerals in the accompanying drawings are indicated in parentheses, but it must be noted that the invention is not limited to the illustrated embodiments alone.